Page 16 - Triwatt Custom Lead Amp Builder's Guide

Introduction to Vacuum Tubes and Common Terms

Reprinted with permission from Aron from diystompboxes.com
Here are a few terms that you may see online when referencing tube schematics. Like distortion
pedals, tube circuits seemingly have their own language! I present this knowledge in the hopes that
it may help you decipher the interesting life of tubes! :-) Below, is a picture and a very simplistic
view of a tube stage.

As you can see above, in this tiny snippet
of a tube schematic, the terms you commonly see are there in this triode stage example.
Plate - the plate is usually connected to a plate resistor which is usually connected to the B+ or
power supply voltage. Typical Plate Resistor values are 100K, 150K, 220K. Larger values equal
more gain.
The Grid is where the signal enters the tube.
The Cathode is usually connected to a cathode resistor which usually goes to ground. The cathode
resistor, along with the Plate resistor, control the gain of the tube stage. Typical values are
anywhere from 100 ohms to 10K. Smaller values = more gain.
It is common to see a cathode bypass cap connected in parallel with the cathode resistor. By
altering the values of the cathode resistor and cathode bypass cap, it is possible to roll off various
degrees of bass with this triode stage. The cathode resistor and plate resistor control the biasing of
the tube. The cathode bypass cap also gives the stage more gain.
Sometimes you see a capacitor in parallel with the plate resistor, much like the cathode resistor
bypass cap. It is usually a small value (i.e. .001uF) and it rolls off highs in the stage. Sometimes you
see a high frequency roll off cap going from the plate pin to the cathode pin - 350pf->500pf in
value.

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